My first ST reading experience was the novelization of ST:TMP and I was thrilled that it filled in gaps in the movie - luckily, I read the book first! Ditto ST III, which opened in Australia several months after the USA. ST II's novelization also had lots of new stuff.
With the TAS "ST Logs", again I was thrilled that the stories were interconnected with new material (esp. backgrounds for the Aprils, Arex and M'Ress).
So, when it came to original novels, the ones that thrilled me the most were the ones that filled in missing information: prequels, sequels and elaborations on canonical ST. One of my first original novels was "The Fate of the Phoenix", but I only knew of the female Romulan Commander via the Blish novelization of "The Enterprise Incident". Later, I found the first "Phoenix" book and the tapestry was woven closed!
"Ex Machina", "Imzadi", "Strangers from the Sky", "Final Frontier", "My Enemy, My Ally", "Crucible" trilogy, "The Buried Age", "Devil in the Sky", "Andor: Paradigm"... most of my favourites are tightly connected to canonical events or have canonical guest stars. "New Frontier", with its myriad of obscure canonical characters. And Peter David, Greg Cox, David R George III and Christopher Bennett are masters at weaving ST tapestries from the canon.